1. Field of the Invention
The invention concerns a distance sensor that operates contactless, which makes the relative displacement and/or relative position of two opposite objects measureable through field intensity measurement of an electromagnetic field emitted by transmitting coils and detected by means of receiving coils. Specifically the invention concerns a position sensor with two coils, the first coil (transmission coil) being fed with a certain frequency so that it emits a constant electromagnetic field and this field being received or detected by means of a second coil (receiving coil).
2. Description of Related Art
Sensors to measure the displacement of two objects relative to each other are generally known. There are different measurement principles, each of which has certain advantages and also disadvantages. The best solution must be chosen, depending on the requirement for accuracy, resolution, temperature dependence, measurement speed, long-term stability, admissible power loss, etc. and the prevailing environmental conditions. Extremely high demands in this respect are involved in position and displacement measurement in precision optics. This involves accuracies in the nanometer and subnanometer range. Two areas of application are of particular significance:
Micropositioning in the production of semiconductors and adjustment of segmented mirrors relative to each other in reflecting telescopes for stellar observation. Several different principles have thus far been used.
Very precise measurements can be accomplished with optical sensors. Capacitive sensors are also very precise. An example of this is described in FR 2 844 048-A1. A major disadvantage in both principles is the dependence of the measurement on effects of moisture and dirt. They are also not suitable in principle for use in reflecting telescopes. Path sensors that operate according to the eddy current loss principle are insensitive to water and dirt, but the temperature dependence of the measurement can only be compensated as precisely as necessary with very high expense.
French unexamined application FR 2 907 211 A1 solves the task of an arrangement of a coil array with a conducting measured object, which is mounted on the opposite mirror segment. The change of inductance is then evaluated, which the measured object produces as a function of its position relative to the individual coils of the array. Measurement can be conducted in three axes with this arrangement. A drawback of the arrangement is the relatively large design and therefore large dependence on mechanical deformations, which can develop, for example, by temperature changes. The temperature-dependent conductivities of the coils and the measurement object also play a non-negligible role.
A transformer principle based on a primary and secondary coil (transmitting and receiving coil) therefore remains as the only possibility.
U.S. Pat. No. 4,816,759 describes for this purpose an arrangement with an oscillator that supplies two transmitting coils connected in series and two receiving coils each of which is completed to an oscillator circuit with a parallel capacitor. One of the two transmitting coils is mounted on the face of a mirror element. The corresponding receiving coil is positioned on the face of the opposite mirror segment. The second transmitting-receiving pair is positioned with reverse transmission-receiving direction, offset at the same height along the faces. The displacement between the transmitting side and the receiving side is evaluated by phase measurement. A zero passage of the phase position of the measured signal is obtained here with plane-parallel agreement of the mirror segments. This principle is essentially suitable for aligning the parallelism of the segments, but provides no additional information concerning other angle and distance dependences of the segments relative to each other.
WO 2007/006910 describes a transformer principle with two primary coils (transmitting coils) connected in series and two opposite secondary coils (receiving coils). The coils are laid out as flat coils, the transmitting and receiving coils each being arranged next to each other and parallel to each other. The transmitting coils are fed in counter-phase with an AC signal. A precise null point recording is obtained with this principle by difference measurement of the two receiver voltages. At the same time, the distance between the two mirror elements can be measured by summing the two receiver voltages. The distance dependence of the measurement of parallelism of the mirror segments relative to each other can be eliminated by forming the ratio of the difference voltage and the sum voltage. What cannot be eliminated, however, is an angle dependence.
A further drawback is the required large surface for the coils, since mechanical deformations, for example, by thermal expansion, play a non-negligible role.
The underlying task of the present invention is to avoid the drawbacks previously encountered in the prior art as much as possible.